Fast radio bursts (FRBs) are observed to be highly polarized. Most have high linear polarization but a small fraction show significant circular polarization. We systematically investigate a variety of polarization mechanisms of FRBs within the magnetar theoretical framework considering two emission sites inside and outside the magnetosphere. For each site, we discuss both intrinsic radiation mechanisms and propagation effects. Inside the magnetosphere, we investigate the polarization properties of both coherent curvature radiation and inverse Compton scattering by charged bunches and conclude that both mechanisms produce 100\% linear polarization at an on-axis geometry but can produce circular polarization if the viewing angle is off axis. The lack of circular polarization for the majority of bursts requires that the bunches have a large transverse dimension size. Resonant cyclotron absorption within magnetosphere may produce high circular polarization if electrons and positrons have an asymmetric Lorentz factor distribution. Outside the magnetosphere, the synchrotron maser emission mechanism in general produces highly linearly polarized emission. Circular polarization would appear at off-beam angles but the flux is greatly degraded and such bursts are not detectable at cosmological distances. Synchrotron absorption in a nebula with ordered magnetic field may reduce the circular polarization degree. Cyclotron absorption in a strongly magnetized medium may generate significant circular polarization. Faraday conversion in a medium with field reversal can convert one polarization mode to another. The two absorption processes require stringent physical conditions. Significant Faraday conversion may be realized in a magnetized dense environment involving binary systems or supernova remnants.
Fast radio bursts (FRBs) are extragalactic radio transients with millisecond duration and extremely high brightness temperature. Very recently, some highly circularly polarized bursts were found in a repeater, FRB 20201124A. The significant circular polarization might be produced by coherent curvature radiation by bunches with the line of sight (LOS) deviating from the bunch central trajectories. In this work, we carry out simulations to study the statistical properties of burst polarization within the framework of coherent curvature radiation by charged bunches in the neutron star magnetosphe…
Fast radio bursts (FRBs) are bright millisecond radio bursts at cosmological distances. Only a small fraction of FRBs apparently repeat. Polarization, a fundamental property of electromagnetic signals, often carries critical information about the radiation processes, the environment, and the intervening medium of FRBs. Here we report circular polarization detections of two active repeating FRBs, namely FRBs 20121102A and 20190520B, with the Five-hundred-meter Aperture Spherical radio Telescope. We detect circular polarization in both active repeating FRBs, which increases the number of repeati…
We report polarization properties for eight narrowband bursts from FRB 121102 that have been re-detected in a high-frequency (4-8 GHz) Breakthrough Listen observation with the Green Bank Telescope, originally taken on 2017 August 26. The bursts were found to exhibit nearly 100% linear polarization, Faraday rotation measures (RM) bordering 9.3$\times$10$^4$ rad-m$^{-2}$, and stable polarization position angles (PA), all of which agree with burst properties previously reported for FRB 121102 at the same epoch. We confirm that these detections are indeed physical bursts with limited spectral occu…
Linear and circular polarizations of gamma-ray bursts (GRBs) have been detected during recent years. We adopt a simplified model to investigate GRB polarization characteristics in this paper. A compressed two-dimensional turbulent slab containing stochastic magnetic fields is considered, and jitter radiation can produce the linear polarization under this special magnetic field topology. Turbulent Faraday rotation measure (RM) of this slab makes strong wavelength-dependent depolarization. The jitter photons can also scatter with those magnetic clumps inside the turbulent slab, and a nonzero var…
The curvature radiation is applied to the explain the circular polarization of FRBs. Significant circular polarization is reported in both apparently non-repeating and repeating FRBs. Curvature radiation can produce significant circular polarization at the wing of the radiation beam. In the curvature radiation scenario, in order to see significant circular polarization in FRBs (1) more energetic bursts, (2) burst with electrons having higher Lorentz factor, (3) a slowly rotating neutron star at the centre are required. Different rotational period of the central neutron star may explain why som…
Fast radio bursts (FRBs) are millisecond-duration radio transients of unknown origin. Two possible mechanisms that could generate extremely coherent emission from FRBs invoke neutron star magnetospheres or relativistic shocks far from the central energy source. Detailed polarization observations may help us to understand the emission mechanism. However, the available FRB polarization data have been perplexing, because they show a host of polarimetric properties, including either a constant polarization angle during each burst for some repeaters or variable polarization angles in some other app…
Polarization can serve as a probe of the radiation mechanism and magnetic field (MF) configuration in gamma-ray bursts (GRBs). In the case of constant MF, the synchrotron polarization in the prompt phase of GRBs has been widely studied. In this paper, we consider the case of the decaying MF. We calculate the time-averaged and instantaneous synchrotron polarizations in a pulse for different viewing angles and for the large-scale decaying MF model, which can explain the so-called Band spectrum. We find that the on-axis time-averaged polarization degree (PD) in the energy band of 50-500 keV for t…
The late afterglow of gamma-ray burst is believed to be due to progressive deceleration of the forward shock wave driven by the gamma-ray burst ejecta propagating in the interstellar medium. We study the dynamic effect of interstellar turbulence on shock wave propagation. It is shown that the shock wave decelerates more quickly than previously assumed without the turbulence. As an observational consequence, an earlier jet break will appear in the light curve of the forward shock wave. The scatter of the jet-corrected energy release for gamma-ray burst, inferred from the jet-break, may be partl…
We report early optical linear polarization observations of two gamma-ray bursts made with the MASTER robotic telescope network. We found the minimum polar- ization for GRB150301B to be 8% at the beginning of the initial stage, whereas we detected no polarization for GRB150413A either at the rising branch or after the burst reached the power-law afterglow stage. This is the earliest measurement of the polarization (in cosmological rest frame) of gamma-ray bursts. The primary intent of the paper is to discover optical emission and publish extremely rare (unique) high- quality light curves of th…
Multi-wave band synchrotron linear polarization of gamma-ray burst (GRB) afterglows is studied under the assumption of an anisotropic turbulent magnetic field with a coherence length of the plasma skin-depth scale in the downstream of forward shocks. We find that for typical GRBs, in comparison to the optical polarization, the degree of radio polarization shows a similar temporal evolution but a significantly smaller peak value. This results from differences in observed intensity image shapes between the radio and optical bands. We also show that the degree of the polarization spectrum undergo…
The repeating FRB source, FRB 20201124A, was found to be highly active in March and April 2021. We observed the source with the Effelsberg 100-m radio telescope at 1.36 GHz on 9 April 2021 and detected 20 bursts. A downward drift in frequency over time is clearly seen from the majority of bursts in our sample. A structure-maximizing dispersion measure (DM) search on the multi-component bursts in our sample yields a DM of 411.6$\pm$0.6 pc/cm$^3$. We find that the rotation measure (RM) of the bursts varies around their mean value of -605 rad/m$^2$ with a standard deviation of 11.1 rad/m$^2$. Thi…
Jet precessions are widely involved in astrophysical phenomena from galaxies to X-ray binaries and gamma-ray bursts (GRBs). Polarization presents a unique probe of the magnetic fields in GRB jets. The precession of GRBs relativistic jets will change the geometry within the observable emitting region of the jet, which can potentially affect the polarization of the afterglow. In this paper, we take into account jet precession to study the polarization evolution and corresponding light curves in GRB early optical afterglows with ordered and random magnetic field geometries. We find that the jet p…
The MeV spectral peak of gamma-ray bursts (GRBs) is best explained as photospheric emission from a dissipative relativistic jet. The observed non-blackbody spectrum shows that sub-photospheric dissipation involves both thermal plasma heating and injection of nonthermal particles, which quickly cool through inverse Compton scattering and emission of synchrotron radiation. Synchrotron photons emitted around and above the photosphere are predicted to dominate the low-energy part of the GRB spectrum, starting from roughly a decade in energy below the MeV peak. We show that this leads to a unique p…
Synchrotron radiation and Compton scattering are widely accepted as the most likely emission mechanisms of some astrophysical phenomena, such as gamma-ray bursts (GRBs) and active galactic nuclei (AGNs). The measurement on polarization of photons provides a useful tool to distinguish different emission mechanisms and structures of the emission region. Based on the differential cross section of a polarized photon scattered by an unpolarized electron of any initial momentum, we derive analytical formula of polarization for beamed photons scattered by isotropic electrons with a power law distribu…
We study milliarcsecond-scale properties of the persistent radio counterpart to FRB 121102 and investigate the spectro-polarimetric properties of a bright burst. For the former, we use European VLBI Network (EVN) observations in 2017 at 1.7 and 4.8 GHz. For the latter, we re-analyse the 1.7-GHz data from the 100-m Effelseberg telescope taken in 2016. These observations predate other polarimetric studies of FRB 121102, and yield the highest burst Faraday rotation measure (RM) to date, RM = 1.27*10^5 rad m^-2, consistent with the decreasing RM trend. The fractional polarization of the burst emis…
We report that the optical polarization in the afterglow of GRB 091208B is measured at t = 149 - 706 s after the burst trigger, and the polarization degree is P = 10.4% +/- 2.5%. The optical light curve at this time shows a power-law decay with index -0.75 +/- 0.02, which is interpreted as the forward shock synchrotron emission, and thus this is the first detection of the early-time optical polarization in the forward shock (rather than that in the reverse shock reported by Steele et al. (2009). This detection disfavors the afterglow model in which the magnetic fields in the emission region ar…
Gamma-ray bursts are the strongest explosions in the Universe since the Big Bang, believed to be produced either in forming black holes at the end of massive star evolution or merging of compact objects. Spectral and timing properties of gamma-ray bursts suggest that the observed bright gamma-rays are produced in the most relativistic jets in the Universe; however, the physical properties, especially the structure and magnetic topologies in the jets are still not well known, despite several decades of studies. It is widely believed that precise measurements of the polarization properties of ga…
The X-ray afterglows of almost one half of gamma-ray bursts have been discovered to have a shallow decay phase by the {\em Swift} satellite, whose origin remains mysterious. Two main models have been proposed to explain this phase, relativistic wind bubbles (RWBs) and structured ejecta, which could originate from millisecond magnetars and rapidly-rotating black holes, respectively. Based on these models, we here investigate polarization evolution in the shallow decay phase of X-ray and optical afterglows. We find that in the RWB model, a significant bump of the polarization degree evolution cu…
In this paper we first review the results obtained by the INTEGRAL mission in the domain of Gamma-Ray Bursts (GRBs), thanks to the INTEGRAL Burst Alert System, which is able to deliver near real-time alerts for GRBs detected within the IBIS field of view. More than 120 GRBs have been detected to date and we summarize their properties here. In the second part of this review we focus on the polarimetric results obtained by IBIS and SPI on GRBs and Galactic compact objects.
A high-degree of polarization of gamma-ray burst (GRB) prompt emission has been confirmed in recent years. In this paper, we apply jitter radiation to study the polarization feature of GRB prompt emission. In our framework, relativistic electrons are accelerated by turbulent acceleration. Random and small-scale magnetic fields are generated by turbulence. We further determine that the polarization property of GRB prompt emission is governed by the configuration of the random and small-scale magnetic fields. A two-dimensional compressed slab, which contains stochastic magnetic fields, is applie…
The polarization of fast radio bursts (FRBs), bright astronomical transients, contains crucial information about their environments. We report polarization measurements of five repeating FRBs, the abundant signals of which enable wide-band observations with two telescopes. A clear trend of lower polarization at lower frequencies was found, which can be well characterized by a single parameter rotation-measure-scatter (σRM) and modeled by multi-path scatter. Sources with higher σRM have higher RM magnitude and scattering timescales. The two sources with the most substantial σRM, FRB 20121102A a…
This paper presents an analysis of the dependence of the opacity for high-energy gamma-rays to gamma-gamma absorption by low-energy photons, on the polarization of the gamma-ray and target photons. This process has so far only been considered using the polarization-averaged gamma-gamma absorption cross section. It is demonstrated that in the case of polarized gamma-ray emission, subject to source-intrinsic gamma-gamma absorption by polarized target photons, this may lead to a slight over-estimation of the gamma-gamma opacity by up to ~ 10 % in the case of a perfectly ordered magnetic field. Th…
The propagation-induced circular polarization of fast radio bursts in relativistic plasma
School of Astronomy and Space Science, Nanjing University（南京大学天文与空间科学学院）；Key Laboratory of Modern Astronomy and Astrophysics (Nanjing University), Ministry of Education（教育部现代天体学与天体物理重点实验室（南京大学））
Aims. With the accumulation of polarization data in the gamma-ray burst (GRB) prompt phase, polarization models can be tested. Methods. We predicted the time-integrated polarizations of 37 GRBs with polarization observation. We used their observed spectral parameters to do this. In the model, the emission mechanism is synchrotron radiation, and the magnetic field configuration in the emission region was assumed to be large-scale ordered. Therefore, the predicted polarization degrees (PDs) are upper limits. Results. For most GRBs detected by the Gamma-ray Burst Polarimeter (GAP), POLAR, and Ast…
We present the first vertically resolved hydrodynamic simulations of a laterally propagating, deflagrating flame in the thin helium ocean of a rotating accreting neutron star. We use a new hydrodynamics solver tailored to deal with the large discrepancy in horizontal and vertical length scales typical of neutron star oceans, and which filters out sound waves that would otherwise limit our timesteps. We find that the flame moves horizontally with velocities of order $10^5$ cm s$^{-1}$, crossing the ocean in few seconds, broadly consistent with the rise times of Type I X-ray bursts. We address t…
Spectropolarimetric measurements of gamma-ray burst (GRB) optical afterglows contain polarization information for both continuum and absorption lines. Based on the Zeeman effect, an absorption line in a strong magnetic field is polarized and split into a triplet. In this paper, we solve the polarization radiative transfer equations of the absorption lines, and obtain the degree of linear polarization of the absorption lines as a function of the optical depth. In order to effectively measure the degree of linear polarization for the absorption lines, a magnetic field strength of at least $10^3$…
We report on the polarization properties of two fast radio bursts (FRBs): 151230 and 160102 discovered in the SUrvey for Pulsars and Extragalactic Radio Bursts (SUPERB) at the Parkes radio telescope. FRB 151230 is observed to be 6 $\pm$ 11% circularly polarized and 35 $\pm$ 13 % linearly polarized with a rotation measure (RM) consistent with zero. Conversely, FRB 160102 is observed to have a circular polarization fraction of 30 $\pm$ 11 %, linear polarization fraction of 84 $\pm$ 15 % for RM = -221(6) rad m$^{-2}$ and the highest measured DM (2596.1 $\pm$ 0.3 pc cm$^{-3}$) for an FRB to date. …
A comprehensive understanding of Gamma Ray Bursts (GRBs) has been elusive due to the variety of questions surrounding the radiation mechanism at play in these events. Polarization measurements of GRBs can heavily constrain the relevant radiation mechanisms and the structure of the GRB jet; however, there is a limited number of theoretical predictions that observed GRB polarizations can be compared against. Here, we conduct radiative transfer calculations of a set of two dimensional relativistic hydrodynamic long GRB (LGRB) jet simulations, of a constant and a variable jet, using the Monte Carl…
Recent polarization measurements of fast radio bursts (FRBs) provide new insights on these enigmatic sources. We show that the nearly 100% linear polarization and small variation of the polarization position angles (PAs) of multiple bursts from the same source suggest that the radiation is produced near the surface of a strongly magnetized neutron star. As the emitted radiation travels through the magnetosphere, the electric vector of the X-mode wave adiabatically rotates and stays perpendicular to the local magnetic field direction. The PA freezes at a radius where the plasma density becomes …
Time-resolved polarizations carry more physical information about the source of gamma-ray bursts (GRBs) than the time-integrated ones. Therefore, they give more strict constrains on the models of GRB prompt phase. Both time-resolved and time-integrated polarizations are considered here. The model we use is the synchrotron emission in a large-scale ordered aligned magnetic field. Time-resolved polarizations of GRB prompt phase are derived with the corresponding time-resolved energy spectra. We found the time-integrated PDs calculated with two methods are similar. So it is convenient to estimate…
Afterglows of gamma-ray bursts (GRBs) are emitted from expanding forward shocks, which are expected to have magnetic field much stronger than the interstellar field, although the origin of the field is a long-standing problem. Two field amplification mechanisms, plasma kinetic instabilities and magnetohydrodynamic instabilities, have been discussed so far. The coherence length scales of the fields amplified by these two processes are different by $7-10$ orders of magnitudes, and the polarimetric observations may distinguish them. We construct a semi-analytic model of the forward shock afterglo…
Fast radio bursts (FRBs) are a new kind of extragalactic radio transients. Some of them show repeating behaviors. Recent observations indicate that a few repeating FRBs (e.g., FRB 121102) present time--frequency downward drifting patterns and nearly 100$\%$ linear polarization. Following the model of \citet{dai 2016} who proposed that repeating FRBs may originate from a slowly-rotating, old-aged pulsar colliding with an asteroid belt around a stellar-mass object, we focus on the prediction of time--frequency drifting and polarization. In this scenario, the frequency drifting is mainly caused b…
Constraining the near-source relativistic wind medium using Fast Radio Burst circular polarization data
University of Texas at Austin（德克萨斯大学奥斯汀分校）；The Open University of Israel（以色列开放大学）；Astrophysics Research Center of the Open university (ARCO)（以色列开放大学天体物理研究中心）；The George Washington University（乔治华盛顿大学）
Random Polarization Position Angle Behaviors across Bursts of Repeating Fast Radio Bursts
State Key Laboratory of Radio Astronomy and Technology, National Astronomical Observatories, CAS（中国科学院国家天文台射电天文与技术重点实验室）；School of Astronomy and Space Science, University of Chinese Academy of Sciences（中国科学院大学天文与空间科学学院）；National Astronomical Observatories, Chinese Academy of Sciences（中国科学院国家天文台）；Department of Astronomy, Beijing Normal University（北京师范大学天文学系）；The Nevada Center for Astrophysics, University of Nevada, Las Vegas（内华达大学拉斯维加斯分校天体物理中心）；Department of Physics and Astronomy, University of Nevada, Las Vegas（内华达大学拉斯维加斯分校物理与天文系）
A long duration gamma-ray burst, GRB 160530A, was detected by the Compton Spectrometer and Imager (COSI) during the 2016 COSI Super Pressure Balloon campaign. As a Compton telescope, COSI is inherently sensitive to the polarization of gamma-ray sources in the energy range 0.2-5.0 MeV. We measured the polarization of GRB 160530A using 1) a standard method (SM) based on fitting the distribution of azimuthal scattering angles with a modulation curve, and 2) an unbinned, maximum likelihood method (MLM). In both cases, the measured polarization level was below the $99\%$ confidence minimum detectab…
The degree and the temporal evolution of linear polarization in the prompt and afterglow emission of gamma-ray bursts is a very robust diagnostic of some key features of gamma-ray bursts jets and their micro and macro physics. In this contribution, I review the current status of the theory of polarized emission from GRB jets during the prompt, optical flash, and afterglow emission. I compare the theoretical predictions to the available observations and discuss the future prospect from both the theoretical and observational standpoints.
The fate and observable properties of gamma-ray burst jets depend crucially on their interaction with the progenitor material that surrounds the central engine. We present a semi-analytical model of such interaction, which builds upon several previous analytical and numerical works, aimed at predicting the angular distribution of jet and cocoon energy and Lorentz factor after breakout, given the properties of the ambient material and of the jet at launch. Using this model, we construct synthetic populations of structured jets, assuming either a collapsar (for long gamma-ray bursts -- LGRBs) or…
We report the first detection of radio polarization of a GRB afterglow with the first intensive combined use of telescopes in the millimeter and submillimeter ranges for GRB171205A. The linear polarization degree in the millimeter band at the sub-percent level ($0.27 \pm 0.04\%$) is lower than those observed in late-time optical afterglows (weighted average of $\sim 1\%$). The Faraday depolarization by non-accelerated, cool electrons in the shocked region is one of possible mechanisms for the low value. In this scenario, larger total energy by a factor of $\sim 10$ than ordinary estimate witho…
The binary neutron star (BNS) merger event GW170817 clearly shows that a BNS merger launches a short Gamma-Ray Burst (sGRB) jet. Unlike collapsars, where the ambient medium is static, in BNS mergers the jet propagates through the merger ejecta that is expanding outward at substantial velocities ($\sim 0.2c$). Here, we present semi-analytic and analytic models to solve the propagation of GRB jets through their surrounding media. These models improve our previous model by including the jet collimation by the cocoon self-consistently. We also perform a series of 2D numerical simulations of jet pr…
Depolarization Induced by Rapid Polarization Angle Swings: A Common Feature of Pulsars and Fast Radio Bursts?
University of Science and Technology of China（中国科学技术大学）；Tsung-Dao Lee Institute & School of Physics and Astronomy, Shanghai Jiao Tong University（上海交通大学李政道理论与计算科学中心及物理与天文学院）；Purple Mountain Observatory, Chinese Academy of Sciences（中国科学院紫金山天文台）
Here, we report on the detection and verification of Fast Radio Burst FRB 180301, which occurred on UTC 2018 March 1 during the Breakthrough Listen observations with the Parkes telescope. Full-polarization voltage data of the detection were captured--a first for non-repeating FRBs--allowing for coherent de-dispersion and additional verification tests. The coherently de-dispersed dynamic spectrum of FRB 180301 shows complex, polarized frequency structure over a small fractional bandwidth. As FRB 180301 was detected close to the geosynchronous satellite band during a time of known 1-2 GHz satell…
We report the earliest-ever detection of optical polarization from a GRB forward shock (GRB 141220A), measured $129.5-204.3\,$s after the burst using the multi-colour RINGO3 optical polarimeter on the 2-m fully autonomous robotic Liverpool Telescope. The temporal decay gradient of the optical light curves from $86\,$s to $\sim 2200\,$s post-burst is typical of classical forward shocks with $α= 1.091 \pm 0.008$. The low optical polarization $P_{BV} = 2.8_{- 1.6} ^{+ 2.0} \, \%$ (2$σ$) at mean time $\sim 168\,$s post-burst is compatible with being induced by the host galaxy dust ($A_{V, {\rm HG}…
